How to quickly install Mattermost on Digital Ocean

Mattermost is a popular free software Slack alternative. Slack can get really expensive really quickly for organizations of any sufficient size, so you either bare with the extremely limited free plan (which holds your data "hostage"), or you switch over to a free alternative like Mattermost. With this, instead of forking over cash for Slack, you can run your own for as cheaply as $5 a month.